Research ArticleProductivity Evaluation for Long Horizontal Well Test in Deep-Water Faulted Sandstone Reservoir

Zhiwang Yuan ,1,2 Li Yang,2 Yingchun Zhang,2 Rui Duan,2 Xu Zhang,2 Yihua Gao,2

and Baoquan Yang2

1School of Energy Resources, China University of Geosciences (Beijing), Beijing 100083, China2CNOOC Research Institute Co., Ltd., Beijing 100028, China

Correspondence should be addressed to Zhiwang Yuan; shiyouxuezi@126.com

Received 22 July 2020; Revised 23 August 2020; Accepted 2 September 2020; Published 22 September 2020

Academic Editor: Guanglong Sheng

Copyright © 2020 Zhiwang Yuan et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

For deep-water faulted sandstone reservoirs, the general practice is to design long horizontal wells improving well productivity.During the project implementation stage, well tests are performed on all drilled wells to evaluate well productivity accurately.Furthermore, multisize chokes are often utilized in a shorten test time for loosen formation, high test cost, and high wellproductivity. Nevertheless, the conventional productivity evaluation approach cannot accurately evaluate the well testproductivity and has difficulty in determining the underneath pattern. As a result, the objective of this paper is to determine aproductivity evaluation method for multisize chokes long horizontal well test in deep-water faulted sandstone reservoir. Thisapproach introduces a productivity model for long horizontal wells in faulted sandstone reservoir. It also includes thedetermination of steady-state test time and the productivity evaluation method for multisize chokes. In this paper, the EGINAOilfield, a deep-water faulted sandstone reservoir, located in West Africa was chosen as the research target. Based on Renardand Dupuy’s steady-state equation, the relationship between the productivity index per meter and the length of horizontalsection was derived. Consequently, this relationship is used to determine the productivity pattern for long horizontal wells withthe same geological features, which can provide more accurate productivity evaluations for tested wells and forecast the wellproductivity for untested wells. After implementing this approach on the EGINA Oilfield, the determined relationship is capableto accurately evaluate the test productivity for long horizontal wells in reservoirs with similar characteristics and assist inexamination and treatment for horizontal wells with abnormal productivity.

1. Introduction

Since the 1990s, deep-water turbidite sandstone reservoirbecomes a hotspot for oil and gas exploration and develop-ment. Major oil and gas deep-water turbidite reservoirs havebeen found in places such as the Gulf of Mexico Basin inNorth America, the Campos Basin in South America, theNiger Delta in West Africa, and the South China Sea [1].

The drilling and well completion cost for a productionwell in deep-water oilfields is nearly 100 million US dollars.As a result, the general strategy in the development ofdeep-water oilfields is to drill less well and maintain a higherplateau production rate. For deep-water faulted oilfields, thegeneral practice is to design long horizontal wells improvingwell productivity. During the project implementation stage,

well tests are performed on all drilled wells to evaluate wellproductivity accurately. Thus, subsequent measurementscan be taken if the well test productivity is lower thanexpected. Considering the loosen reservoir formation andhigh well productivity, multisize chokes were adopted to pre-vent sand production during the test process. Simulta-neously, the well test time is usually kept in a very shortdue to test cost, crude oil storage, and other factors, whichmakes well productivity evaluation difficult. Therefore, theconventional productivity evaluation approach cannot accu-rately evaluate the well test productivity and has difficulty indetermining the productivity pattern. Thus, it is necessary toestablish an accurate productivity evaluation method for longhorizontal wells using multisize chokes in deep-water faultedsandstone reservoirs. Currently, the Lower Congo and the

HindawiGeofluidsVolume 2020, Article ID 8845270, 10 pageshttps://doi.org/10.1155/2020/8845270

Niger Delta Basin are the attraction points for the explorationand development of deep-water turbidite reservoirs. Theyhave been studied by petroleum engineers all over theworld, which include their deposition mechanism [2, 3],deposition model [3, 4], deposition characteristics, evolu-tion pattern [1], channel distribution and configurationcharacteristics [5–7], connectivity mode and connectivity[8, 9], and water-cut rising mechanism and water injectionoptimization method [10]. In addition, the results of oilwell productivity evaluation are mainly concentrated inthe time correction coefficient [11–13], interlayer interfer-ence coefficient [14], and equivalent test time [15] in theshallow water oilfields, while in deep-water faulted sand-stone reservoirs, the test productivity evaluation is limitedin aspect of the productivity test process and on-site oper-ations [16]. There are only few studies on the productivityevaluation method with multisize chokes for long horizon-tal wells.

The EGINA Oilfield, a deep-water faulted sandstonereservoir, located in West Africa was evaluated in thispaper. A productivity evaluation approach was establishedfor long horizontal wells with multisize chokes in deep-

water faulted sandstone reservoirs. The method is com-posed of the establishment of productivity model for longhorizontal wells in faulted sandstone reservoirs, the deter-mination of steady-state test time with multisize chokes,and the productivity evaluation method with multisizechokes. Based on Renard and Dupuy’s steady-state equa-tion, the relationship between the productivity index permeter and the length of horizontal section was deter-mined, and the pattern for test productivity in long hori-zontal wells with the similar geological features isderived. This finding can assist better understanding tothe productivity evaluation for tested wells and forecastthe well productivity for untested wells. By practicing inthe EGINA Oilfield, the productivity evaluation approachand productivity pattern are proven to be reliable.

2. EGINA Oilfield Overview

The EGINA Oilfield is located in the deep-water contractarea of OMLl130 in southern Nigeria, West Africa, about160 km away from the Port Harcourt in the north (seeFigure 1). The oilfield water depth is ranging from 1150 to

OPL 215

50 km OPL 246OML 130Egina &

Akpo

OPL 221OPL 222USAN

OPL 223

Figure 1: EGINA field location [18].

hL

zy

x

ZW

Figure 2: Physical model of a horizontal well in faulted sandstone reservoir.

2 Geofluids

1750m [16, 17]. It is operated by the TOTAL on behalf of theNigerian National Petroleum Corporation (NNPC-Nigeria),the South Atlantic Petroleum Limited (SAPETRO-Nigeria),the PETROBRAS (Brazil), and the China National OffshoreOil Corporation (CNOOC China).

The EGINA Oilfield is a faulted anticline with compli-cated geological characters. The sedimentary facies of the res-

ervoir is deep-water turbidite fan, and the lithology of thereservoir is fine-coarse sandstone which is mainly composedby quartz and feldspar. The reservoir is shallow buried, loosecemented, and weak compacted leading to medium-highporosity and high permeability. Primary intergranular poreforms the main reservoir spaces. The formation fluid ismedium oil with low viscosity.

1200

1050

900

750

Oil

rate

(m3 /d

)

Chok

e siz

e (/6

4″)

600

450

300

150

00 5 10 15

Time (h)20 25 30

64

56

48

40

32

24

16

8

0

Liquid rateChoke size

Figure 3: P-4 productivity test curve.

I-3 I-2

P-3

P-4

I-1Scale1 km

Oil-bearingarea

Oil-bearingboundary

Fault Sealingfault

Shelteringfault

Horizontalproducer

Horizontalinjector

Completionsection 1/2

Figure 4: Well location of P-4.

3Geofluids

The EGINA Oilfield is developed using subsea wellheadand injection manifolds connected to an FPSO. The produc-tion has commenced from December 2018.

3. Purpose and Procedure of Productivity WellTest for Long Horizontal Well in Deep-WaterSandstone Oilfield

3.1. Test Purpose. The main purpose of the oil well productiv-ity test is to:

(1) The first purpose is removing completion fluids andbreaking down the mud-cake into small chips whichcan pass through the screen. We need to clean the entirescreen length down to low water cut prior to their tie-back to the subsea manifolds and to the FPSO [16]

(2) Another purpose of the productivity test is to collectfluid samples and obtain well/reservoir dynamicparameters relating to reservoir characteristics andcompletion efficiency (KH, skin, flow boundary, andproduction index), which help to reduce the uncer-tainty of productivity and fluid properties in the earlystage of oilfield production

To make sure the oil field is reaching the production pla-teau as expected, the productivity test and evaluation of eachwell are carried out after the well is drilled and before it com-mences production.

3.2. Test Procedure. The productivity test design is completedby a well test software. Based on the expected range of reser-voir physical parameters, a variety of test schemes aredesigned, and the optimized scheme is selected to determinethe durations of well cleaning up and pressure buildup.

Small variations exist between each well test scheme. Forunconsolidated sandstone reservoir, in order to prevent sandproduction, the general productivity test procedure is as fol-lows: increase the choke sizes step by step by 4/64″ and

increase the daily oil production by no more than specificdrawdown (4 bar for frac-packs and 2 bar for SAS and ES)or 300m3/d. Before switching to the next choke size, the pres-sure of each choke size shall be kept stable for at least 2 hours,and sand production is monitored simultaneously [16].

4. Productivity Evaluation Method of LongHorizontal Well Test in FaultedSandstone Oilfield

4.1. Productivity Model of Long Horizontal Wells in FaultedSandstone Reservoirs with Different Boundaries. The assump-tions are as follows: (1) the fluid in the formation is single-phase fluid with low compressibility; (2) the effect of gravityand capillary force is neglected; (3) the pressure in all partsof the formation is the initial reservoir pressure before theproductivity well test; (4) the reservoir is homogeneous, andthe fluid flow achieves Darcy seepage; and (5) the top andbottom boundaries of the formation are impermeable.

Figure 2 is a physical model of a horizontal well in afaulted sandstone reservoir. Assuming the reservoir thicknessis h and the horizontal well length of L from the bottomboundary ZW , the reservoir permeability in the X directionis equal to ones in the Y direction, which is KX = KY = KH .Under different boundary conditions, the expression ofdimensionless bottom hole flowing pressure for horizontalwells without wellbore storage and skin with a fixed produc-tion rate Q is [19, 20]:

pD = π

LD

ðtD0GxD xD, τð Þ ⋅GyD yD, τð Þ ⋅GzD zD, τð Þdτ, ð1Þ

where CD is the dimensionless wellbore storage coeffi-cient; S is the skin factor; pD is the under different boundariesconditions, dimensionless bottom hole flowing pressure forhorizontal wells without wellbore storage, and skin with afixed production rate Q; Q is the daily oil production rate,m3/d; LD is the dimensionless length of horizontal section

10

1

0.1

0.01

Del

ta P

and

deriv

ativ

e, ba

r

0.0001 0.001 0.01 0.1Time (h)

1 10 100

Actual pressureActual pressure derivative

Model’s pressureModel’s pressure derivative

Figure 5: Interpretation chart of P-4 pressure buildup.

4 Geofluids

for horizontal well; tD is the dimensionless time; xD, yD, andzD are the dimensionless coordinates in the X direction, Ydirection, and Z direction, respectively; and GxDðxD, τÞ, GyDðyD, τÞ, and GzDðzD, τÞ are the green function in the X direc-tion, Y direction, and Z direction, respectively;

Under different boundary conditions, GxD, GyD, and GzDcan be expressed as follows [19, 20]:

(1) Under different boundaries conditions, GxDðxD, τÞcan be expressed as follows:

(i) Both boundaries are sealing in the X direction

GxD = 2LDxeD

1 + 2〠∞

n=1

1nexp −

n2π2tDx2eD

� �sin LD

xeD

� �cos

"

� nπxwDyeD

� �cos nπxD

xeD

� ��:

ð2Þ

(ii) Both boundaries are constant pressure in the Xdirection

GxD = 4π〠∞

n=1

1nexp −

n2π2tDx2eD

� �sin nπxwD

xeD

� �sin

�

� nπLDxeD

� �sin nπxD

xeD

� ��:

ð3Þ

(iii) Mixed boundaries in X direction(sealing at xD = 0,constant at xD = xeD)

GxD = 8π〠∞

n=0

12n + 1 exp −

2n + 1ð Þ2π2tD4x2eD

" #sin 2n + 1ð ÞLD

xeD

� �cos

"

� 2n + 1ð ÞπxD2xeD

� �cos 2n + 1ð ÞπxwD

2xeD

� ��:

ð4Þ

(2) Under different boundaries conditions, GyDðyD, τÞcan be expressed as follows:

(i) Both boundaries are sealing in the Y direction

GyD = 1yeD

1 + 2〠∞

n=1exp −

n2π2tDy2eD

� �cos2 nπywD

yeD

� �" #: ð5Þ

(ii) Both boundaries are constant pressure in the Ydirection

GyD = 2yeD

〠∞

n=1exp −

n2π2tDy2eD

� �sin2 nπxwD

yeD

� �" #: ð6Þ

(iii) Mixed boundaries in the Y direction (sealing at yD= 0, constant at yD = yeD)

GyD = 2yeD

〠∞

n=1exp −

n2π2tDy2eD

� �cos2 nπxwD

yeD

� �" #: ð7Þ

(3) In the case of no gas cap and bottom water in the Zdirection, GzDðzD, τÞ can be expressed as follows:

GzD = 1 + 2〠∞

n=1exp −

n2π2tDhD

� �cos2 nπzw

h

� �: ð8Þ

Considering wellbore storage and skin, the dimensionlessbottom hole flowing pressure of horizontal well can beexpressed as follows [19, 20]:

pwD = upD + S/LDu 1 + CDu upD + S/LDð Þ½ � , ð9Þ

where pwD is the under different boundaries conditions,dimensionless bottom hole flowing pressure for horizontalwells with wellbore storage, and skin with a fixed productionrate Q in the Laplace space and u is the Laplace transforma-tion constant.

The Laplace numerical inversion of equation (9) is car-ried out to obtain the relationship between bottom hole flow-ing pressure and time, which can predict the productivity oflong horizontal well with time in faulted sandstone reservoirunder different boundary conditions.

4.2. Steady-State Time Determination of Multisize ChokesTest in Long Horizontal Well. For the well test with multi-size chokes, it is necessary to determine whether the pro-ducer is steady or pseudo steady-state at all choke sizesand therefore select the appropriate methods to evaluatethe test productivity. For a complex faulted sandstone res-ervoir with closed boundaries, a mathematical model canbe established based on the reservoir parameters inter-preted from the producer’s pressure buildup curve and

5Geofluids

the distribution characteristics of the reservoir and faults.Then, by simulating the well productivity test, the requiredtime to reach the steady-state can be calculated.

4.3. Productivity Evaluation Method of Multisize Chokes Testin Long Horizontal Well Test. If well test with multiplechoke sizes reach stable or pseudo steady-state, the dataof these choke sizes can be used to derive the relationshipbetween the bottom hole flowing pressure and the produc-tion rate. From equation (10), the reciprocal of the lineargradient is the productivity index of the test well.

pw = pe −qJh, ð10Þ

where pw is the bottom hole flowing pressure, MPa; pe isthe formation pressure, MPa; q is the daily oil production,m3/d; and Jh is the productivity index of horizontal well,m3/d/MPa.

If the well tests with multiple choke sizes do not reach sta-ble or pseudo steady-state, the test data of last choke size isused to calculate the transient productivity index, and the

75

60

45

30

15

0200 300

Actual dataFitting line

400K/[𝜇ln(4re/L)]

500 600

Prod

uctiv

ity in

dex

per m

eter

,m

3 /(d

MPa

m)

Figure 7: Relationship between steady productivity index per meter and K/½μ ln ð4re/LÞ�.

75

60

45

30

15

0200 300

Actual data

Fitting line

400 500 600

P-3 data before acid stimulation

K/[𝜇ln(4re/L)]

Prod

uctiv

ity in

dex

per m

eter

,m

3 /(d

MPa

m)

Figure 8: Productivity index per meter evaluation of P-3 before acidstimulation.

180

150

120

Prod

uctiv

ity in

dex

per m

eter

,m

3 /(d

MPa

m)

90

60

30700 1000

Actual dataFitting line

1300K/𝜇 (mD/cp)

1600 1900 2200

y = 0.0429x+55.0992R2 = 0.4284

Figure 6: Relationship between transient productivity index per meter and mobility.

6 Geofluids

equivalent test time can be determined by equation (11) [15].If the evaluated equivalent time does not reach stable orpseudo steady-state, the time correction coefficient, the pro-ductivity index at stable or pseudo steady-state divided bythe transient test productivity index, can be obtained by themathematical model established in Section 4.1. As a result,the productivity index at the stable or pseudo steady-state isobtained.

tpe =Np

qh, ð11Þ

where Np is the well test cumulative oil production, m3; qh isthe daily oil production of last choke size, m3/d; and tpe is theequivalent test time, d.

5. Productivity Evaluation Method for Well TestProductivity Pattern of LongHorizontal Wells

To derive the productivity evaluation approach for test pro-ductivity pattern of long horizontal well, Renard andDupuy’s equation [21] is analyzed, as shown in equation (12):

we set

a = L2

12 +

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi14 + 2re

L

� �4s2

4350:5

ð13Þ

where μ is viscosity of the formation crude oil, cp; k is the res-ervoir permeability, mD; h is the reservoir thickness, m; L isthe length of the horizontal section, m; rw is the wellboreradius, m; re is the drainage radius, m; and a, b, c, and d areconstant.

We set b = ðμ/0:543khÞ ln ða +ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 − ðL/2Þ2

q/L/2Þ

,c = ðμ/0:543kLÞ ln ðh/2πrwÞ, and d = a +ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 − ðL/2Þ2

q/L/2,

from equation (13):

d =L/2ð Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1/4 + 2re/Lð Þ4

q+ 1/2

� �0:5+ L/2ð Þ

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1/4 + 2re/Lð Þ4

q− 1/2

� �0:5L/2

=ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi14 + 2re

L

� �4s

+ 12

24

350:5

+ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi14 + 2re

L

� �4s

−12

24

350:5

:

ð14Þ

100

10

1

0.1

0.01

0.0010.0001 0.001

Actual data before acid stimulationModel’s data before acid stimulation

0.01Time (h)

0.1 1 10 100

Del

ta P

and

deriv

ativ

e, ba

r

Figure 9: Interpretation chart of P-3 pressure buildup before acid stimulation.

Jh =q

Pe − Pw= 1

μ/0:543khð Þ ln a +ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 − L/2ð Þ2

q� �/ L/2ð Þ

� �+ μ/0:543kLð Þ ln h/2πrwð Þ

, ð12Þ

7Geofluids

Square the both sides of equation (14):

d2 = 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi14 + 2re

L

� �4s

+ 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi2reL

� �4s

= 2ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi14 + 2re

L

� �4s

+ 2 2reL

� �2≈ 4 2re

L

� �2:

ð15Þ

From equation (15):

d ≈4reL

, ð16Þ

bc= Lh

lna +

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffia2 − L/2ð Þ2

qL/2 /ln h

2πrw

0@

1A: ð17Þ

Substituting equation (16) into equation (17):

bc= Lh

ln 4re/Lln h/2πrw

� �: ð18Þ

For long horizontal wells, the difference betweenln(4re/L) and ln(h/(2πrw)) is small, while the length L of hor-izontal wells is far greater than the reservoir thickness h;therefore:

bc= Lh

ln 4re/Lln h/2πrw

� �> >1: ð19Þ

Thus, constant c can be neglected. Substituting equation(16) into equation (12):

Jom = Jhh

= qPe − Pwð Þh ≈

0:543kμ ln 4re/Lð Þ , ð20Þ

where Jom is the productivity index per meter, m3/d/MPa.

Equation (20) underlines the linear relationship betweenJom and K/½μ ln ð4re/LÞ�. Based on this relationship, the testproductivity pattern of long horizontal oil wells with the sim-ilar geological features is determined, which can guide theproductivity prediction of untested oil wells and improvethe evaluation for the tested oil wells.

6. Applications and Discussions

The EGINA Oilfield adopts the development strategy “lesswells with high production rate.” All drilled oil wells are con-ducted with the productivity test to remove completion fluidsand ensure the accuracy of the test productivity. Due to thehigh well productivity and loosen reservoir formations, thetest procedure increasing the test choke step by step (Section3.2) is adopted to prevent sand production during the pro-ductivity test. At the same time, high test cost in deep-wateroilfield is considered as the well test time is kept very short.For instance, the oil well P-4 was tested using 10 differentchoke sizes with increasing pattern. The maximum daily oilproduction was 1150m3/d, and a total test time was 27.4hours, as shown in Figure 3.

6.1. Well Test Productivity Evaluation with MultisizeChokes for P-4 Well

6.1.1. Determination of Steady-State Time for P-4 Well. Toevaluate the well test productivity of the P-4 well, the first pri-ority is to confirm whether the tests at all choke sizes havereached the steady or pseudo steady-state to select the appro-priate method. P-4 is a horizontal well in a complex faultedsandstone reservoir; the well location map is illustrated inFigure 4. The mathematical model is established based onthe reservoir parameters derived from the pressure buildupcurve of P-4 (see Figure 5) and faults distribution character-istics. Subsequently, this model is used to simulate the proce-dure of well productivity test, and the time calculated for thewell to reach the steady-state is 51 days.

6.1.2. Productivity Evaluation with Multisize Chokes for P-4Well. From Figure 3, the ten choke sizes tested at P-4 wellhave not reached steady-state. The tested data of final chokesize is used to calculate the transient productivity, and thetransient productivity index of P-4 is 3666m3/d/MPa. Theequivalent test time calculated by equation (11) is 9.7 hours,combined with the productivity model in the previous Sec-tion 4.3; the time correction coefficient is 0.41, and thesteady-state productivity index is 1503m3/d/MPa.

6.2. Pattern of Productivity Evaluation for Long HorizontalWells in Faulted Sandstone Oilfields. To derive the patternof well test productivity for long horizontal wells in faultedsandstone reservoir, horizontal wells with similar geologicalfeatures were screened out, and the relationship betweenthe transient productivity index and mobility was analyzed.Based on Figure 6, the linear relationship is not obvious.However, the method introduced in this paper is able todetermine the productivity index of each well at steady-state. As a result, the relationship between productivity indexat steady-state and K/½μ ln ð4re/LÞ� can be found based on

75

60

45

30

15

0200 300 400

K/[𝜇ln(4re/L)]500 600

Actual data

Fitting lineP-3 data after acid stimulation

Prod

uctiv

ity in

dex

per m

eter

,m

3 /(d

MPa

m)

Figure 10: Productivity index per meter evaluation of P-3 after acidstimulation.

8 Geofluids

equation (20), which is a nicely fit (see Figure 7). Therefore,this approach can evaluate the pattern of tested well produc-tivity and predict the productivity of untested wells.

6.3. Examination and Treatment of Horizontal Wells withAbnormal Test Productivity

6.3.1. Examination of Horizontal Wells with Abnormal WellTest Productivity. To ensure the productivity of horizontalwells as expected before production commencement, anexamination of the reliability for the test productivitybecomes a necessary step. To begin with, the method intro-duced in Section 4 is utilized to evaluate the well test produc-tivity of tested wells. Then, the reliability of test result isanalyzed by the productivity trend of tested horizontal wellsmentioned in Section 5. If a well with abnormal test produc-tivity exists, it will be suggested to take corresponding mea-sures. For example, after productivity evaluation of well P-3, it is compared with relationship between productivityindex per meter of horizontal wells at steady-state and K/½μln ð4re/LÞ�. Based on Figure 8, the tested productivity of P-3 is significantly lower than other wells. From the pressurebuildup interpretation of P-3 (see Figure 9), its skin factoris 34, which is obviously too high. Thus, serious reservoir pol-lution near the well bore is suggested to be the cause of thehigh skin factor.

6.3.2. Treatment of Horizontal Wells with Abnormal WellProductivity. Acidizing treatment was carried out to removethe reservoir pollution near the wellbore of P-3. The produc-tivity after acidizing treatment is compared with the relation-ship between the productivity index per meter of testedhorizontal wells at steady-state and the K/½μ ln ð4re/LÞ�.From Figure 10, the productivity is on the trend line, andthe skin factor becomes 0.20 (see Figure 11), indicating thatthe pollution near the wellbore has been removed.

7. Conclusion

(1) A productivity evaluation method for multisizechokes long horizontal wells in deep-water faultedsandstone reservoirs is established to consider loosenreservoir formation, high well productivity, and shortwell test time, which can determine the productivityof long horizontal wells at steady-state

(2) Based on Renard and Dupuy’s steady-state equation,the relationship between the productivity index permeter and the length of horizontal section wasderived. The trend line of test productivity for longhorizontal wells under the same geological featuresis obtained, which can guide to a more accurate pro-ductivity evaluation for tested wells and forecast theproductivity for untested wells

(3) Based on the field practice in the EGINA Oilfield,productivity evaluation and pattern derived fromthe tested results at steady-state can suggest appro-priate treatment method for horizontal wells withabnormal productivity. This method is proven to bereliable and can evaluate the test productivity of longhorizontal wells in similar deep-water faulted sand-stone reservoirs

Data Availability

The data used to support the findings of this study are avail-able from the corresponding author upon request.

Conflicts of Interest

The authors declare no conflict of interest.

100

10

1

0.1

0.010.0001 0.001

Actual data before acid stimulationModel’s data before acid stimulation

0.01Time (h)

0.1 1 10 100

Pres

sure

diff

eren

ce, b

ar

Actual data before acid stimulationModel’s data before acid stimulation

Figure 11: The pressure buildup interpretation comparison of P-3 before and after acid stimulation.

9Geofluids

Authors’ Contributions

Conceptualization was done by Zhiwang Yuan and Li Yang;methodology was done by Zhiwang Yuan; validation wasdone by Zhiwang Yuan and Yingchun Zhang; writing theoriginal draft preparation was done by Zhiwang Yuan; writ-ing the review and editing were done by Zhiwang Yuan,Rui Duan, Xu Zhang, and Yihua Gao.

Acknowledgments

The authors are indebted to Baoquan Yang for sharing theirmeaningful points and discussions. This research was fundedby the National Science and Technology Major Project (grantnumber 2017ZX05032-004).

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